1. Field of the Invention
The invention relates generally to control of a rotating electric machine, and more specifically, to a load responsive speed regulation device and method for an inverter fed variable speed induction motor drive system based on power factor angle estimation and slip control.
2. Description of Related Art
Many applications for electric motors require variable speed motor operation, and to this end, various speed control solutions have been proposed. Induction motors are popular for several reasons, including high robustness, reliability, low price and high efficiency. A typical induction motor includes a stationary member, or stator, that has a plurality of windings disposed therein. A rotating member, or rotor, is situated within the stator to rotate relative thereto. In a three-phase induction motor, for example, a rotating magnetic field is established by applying three-phase sinusoidal alternating voltages to the stator windings. The rotating magnetic field interacts with the rotor windings to effect rotation of the rotor.
Power conversion systems are commonly used to provide the multiphase AC power to the induction motor for variable speed applications. An example of such a power conversion system is a DC-to-AC inverter bridge, which typically includes inverter switching devices connected in a bridge formation between the DC bus lines and output terminals of the power conversion system. The inverter switching devices are controlled to connect the power on the DC bus lines to the system output terminals in a desired pattern such that AC output signals having the desired frequency and amplitude are synthesized from the DC power on the DC bus lines. Various modulation strategies may be employed for controlling the inverter switching devices to deliver power, including sine wave Pulse-Width Modulation (xe2x80x9cPWMxe2x80x9d).
The desired speeds are achieved by setting the excitation frequency and a corresponding voltage. Low cost speed control solutions are often implemented using constant or schedule based volts-per-hertz algorithms, and speed regulation is based on feedback from a tachometer on the motor shaft. Elimination on the tachometer, however, is desirable not only from a cost perspective, but also for reliability reasons. Processing the speed information from tachometers requires extra signal conditioning circuits and takes up additional input-output pins on the processor chip.
Moreover, an induction motor""s speed slips relative to synchronous speed as the load increases. In applications where the load on the motor shaft is varying (for example, a washing machine), it is also important that proper stator voltage be applied to enable efficient operation and avoid over-heating of the motor due to saturation and iron losses. Meeting these requirements requires a load responsive control system that can provide near constant speed operation of the motor and the right amount of the voltage to the stator, thus preventing saturation in the motor core.
Solutions have been proposed for efficient operation of an induction motor based on controlling the power factor of the motor (generally, the power factor is calculated based on the phase difference between the voltage and currents). Such solutions, however, may not be satisfactory in low cost solutions. Additionally, cost effective solutions for a variable speed induction motor drive systems fed by PWM inverter bridges that use simple volts-per-hertz control are not generally available.
The present invention addresses shortcomings associated with the prior art and proposes a low cost solution to achieve the same results.
The present invention provides a load responsive variable speed induction motor drive system and method based on power factor angle sensing and slip control to maintain speed regulation without the need for a tachometer.
In one aspect of the invention, a method for controlling an induction motor is provided. The induction motor has a rotor and a stator with a plurality of phase windings therein to which AC power is applied to cause rotation of the rotor relative to the stator. The method includes sensing the zero-cross angle of a phase current waveform in the phase windings and computing the difference between the sensed current zero-cross angle and a known stator voltage angle to estimate a power factor angle. The estimated power factor angle is compared to a command power factor angle, and the voltage applied to the motor is adjusted in response to the error between the estimated power factor angle and the command power factor angle. The input frequency may also be adjusted in response to the difference between the estimated power factor angle and the desired power factor angle during certain load conditions to keep speed constant.
In accordance with other aspects of the present invention, an induction motor system includes a stator and a rotor situated relative to the stator to rotate relative to the stator. A plurality of phase windings are situated within the stator, and a power source is connected to the windings to output AC power thereto. In exemplary embodiments, a DC-AC inverter provides the AC power to the motor. A controller is connected to the inverter, and is programmed to compare a sensed current zero-cross angle to a predetermined demand voltage angle to estimate a power factor angle. The voltage, and in some embodiments, the frequency of the voltage applied to the motor are adjusted in response to the difference between the estimated power factor angle and a predetermined desired power factor angle.